This schematic is a very functional 2-wide spiral staircase. It has an 8 lx 8w x 12h footprint per iteration. The first inside stair pastes directly in front of your feet.when facing east. It can be stacked as many times as needed to reach a desired height.
Regardless of the torches inside the stairwell, No mobs should be able to spawn inside the stairwell as mobs cannot spawn on normal (non-inverted) stair blocks and bottom slabs.